Passive WiFi: Transmitting data with almost no electricity

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Wireless communication with increasingly negligible energy consumption—it sounds almost too good to be true. And yet the idea is as old as it is simple. All you have to do is factor out the analog power guzzlers.

Whether it comes to industry, households, buildings or cities: They should all be smart. That definitely calls for communication between every device possible. Even coffee machines, towel dispensers and toothbrushes are no exception. However, costs and energy consumption are still preventing IoT applications that could increase utility considerably. And that despite particularly frugal transmission standards such as Bluetooth Low Energy or Zigbee.

A team from the University of Washington, on the other hand, has been trying for several years to affordably give all devices no matter how tiny highly energy-efficient connectivity. In doing so, they are turning to a technology that served as a listening device that the Soviet Union hid in a gift to the American Ambassador in 1945. The so-called “thing” basically consisted of an acoustic membrane and an antenna. Without its own power source, all it did was change the signal of another transmitter to send information.

It is not known what distances the “thing” was able to bridge during the seven years of its covert use. Now, more than seventy years later, engineers in Washington are claiming to have set a world record with a range of 2.8 kilometers. That is certainly sufficient for applications in buildings, but also at industrial sites and even in smart cities. The current system worked in an office building with 41 rooms and some 450 square meters of total space. Communication also succeeded from every part of a neighboring vegetable farm that was 4,000 square meters in size.

Passive WiFi with a limited thirst for electricity

At the end of last year, researchers used passive WiFi in a listening device that needed 10,000 times less electricity to transmit than conventional systems. At the time, the range of the passive WiFi was approximately 30 meters and the data rate was 11 megabits per second—eleven times higher than that of Bluetooth.

If you want to use that little electricity, you must separate the analog from the digital technology. After all, although the digital portion of WiFi communication only requires a few microwatts, the portion responsible for the radio-wave carrier signal is very power hungry.

That is why, in passive WiFi, a central device that is connected to a power supply assumes these analog functions for all IoT sensors that communicate via WiFi. The passive WiFi modules in the IoT devices, say for home automation or machine monitoring, only modulate the signal with the message to be transmitted and forward it to commercially available routers, smartphones or computers.

Setting a world record with three components

The “record-breaking configuration” consists of the same three components: A source for the radio signals, the sensor whose data is to be transmitted, and a decoder that decodes the incoming data. Tiny printed batteries are sufficient to transmit the data.

Of course, signals that are that weak are not easy to decode because they are superimposed over non-reflected signals and other “noise”. That calls for a special modulation technique that is used in sensor-actuator networks in the industrial sector and in building automation. If the amplitude of the chirp impulse remains constant, this so-called chirp-spread spectrum changes the frequency within a stipulated time between the initial frequency and the final frequency. Doing so increases sensitivity and makes it possible to decode the signal over a greater distance if it is weaker than the noise signal.

Passive WiFi as early as next year

The data transmission chip costs between ten and twenty US cents, which makes it ideal for mass use. Moisture sensors in arable farmland, air-monitor sensors at monitoring stations and temperature sensors in buildings are ideal application scenarios for the technology. Due to the continuous availability of data, it can also be used to set up smart real-time control systems such as those used to control traffic in city centers.

The system is expected to hit the market in the spring of 2018. It is being further developed and operated by Jeeva Wireless, a spin-off of the University of Washington. An investment round early this year generated 1.2 million US dollars to commercialize the technology. That could be money well invested, seeing as MIT’s “Technology Review” magazine has already declared the invention one of the technology breakthroughs of last year.

Passive WiFi (Image: University of Washington)

The new technology could make measuring devices for sports and medicine even smaller and more durable. (Image: University of Washington)